Oscillation generating systems



June 29, 6 C; A. E; BEURTHERET 3,192,354

OSOILLATION GENERATING SYSTEM Filed March 23, 1962 2 Sheets-Sheet 1 //V VE/V TOR C. AE. Beurzfieref W/MW ATTORNEYS June 29, 1965 c. A. E. BEURTHERET 3,192,354

OSGILLATION GENERATING SYSTEM Filed March 23, 1962 2 Sheets-Sheet 2 ATTo RNEYS United States Patent Office 3,192,354 Patented June 29, 1965 3,192,354 OSCILLATION GENERATING SYSTEM Charles Alphonse Emile Beurtheret, St. Germain en Laye,

France, assignor to Compagnie Francaise Thomsonw,

Houston, Paris, France, a corporation of France Filed Mar. 23, 1962, Ser. No. 182,058 Claims priority, application France, Mar. 28, 1961, 857,025, Patent 1,296,598 Claims. (Cl. 219-10.77)

This invention relates to oscillation generating circuits, more particularly of the type used for producing comparatively high-frequency oscillatory energy that is applied to work for causing desired physical and/or chemical modifications in the work. Examples of industrial processes applying oscillation generating circuits of the general class to which the invention relates include the dielectric heating of insulating substances, heat treatment of metal parts, production of high-melting alloys, and many other treatments.

It is a general object of this invention to provide an improved oscillation generating system that will make it possible easily and efficiently to maintain the energy output of the circuit at a controlled, e.g., constant, value despite uncontrolled variations in the impedance of the load, and with little or no variations in the output frequency,

In the operation of oscillator circuits of the class speci fled, serious problems have heretofore been encountered in respect to circuit adjustment, in view of the wide and largely uncontrollable variations in the overall electrical characteristics of the work, both from one treatment to another and throughout a given treatment owing to physical and chemical modifications that the work may be undergoing. Thus, it may often be necessary to use a given system in connection with workpieces differing considerably from one another in their constituent material, their shape and size. Further, during the treatment the workpiece will usually undergo a continuous variation in its resistance as its temperature changes, and in addition such resistance as well as other electrical characteristics of the work material may develop sudden changes in value due to some physical or chemical transformation occurring within the material, such as at the Curie point of magnetic materials.

Such variations in load characteristics result in corresponding variations in the load impedance of the oscillation producing circuit, and require some form of compensatory adjustment in the circuit characteristics if the energy output of the system is to retain its prescribed value. While various means have been suggested and/ or used for maintaining a constant energy output despite variations in load impedance, none has proved entirely satisfactory.

Sometimes a variable-ratio impedance matching transformer is interposed between the circuit and the load, and the ratio is varied e.g. by relatively moving the primary and secondary windings of the transformer or a shorted tertiary control winding thereof. While such a device may give adequate results at relatively low power ratings, at higher energy levels the engineering problems associated with providing adequate electrical insulation and effective heat dissipation in the moving parts of the transformer while traversed by very high currents under high voltages tend to become inseparable.

Another method sometimes used is to provide for varying the output frequency of the oscillations over a Wide range, e.g. in a ratio of 3 to 1, by selectively switching the reactive circuit elements of the system as between series and parallel relationship, or other circuit configurations. This solution is inapplicable at comparatively very high frequencies, is undesirably discontinuous in character, and introduces the further and very serious disadvantage of introducing a large change in output frequency, which detracts from the efficiency of the treatment in the case of many types of processes. It has additionally been proposed to provide a reactance element (e.g. a capacitor) in parallel with the load and switch said element into and out of circuit as the occasion may demand, e.g. on a sudden change in load characteristics for example as a steel workpiece undergoing treatment attains its Curie point. However, the applicability of this device is limited, and further it has been found that at the instant of switching the oscillator will frequently show a tendency to lock in on an unwanted frequency higher than the prescribed frequency.

Objects of this invention therefore are to provide an improved oscillation generating system especially useful in connection with high frequency energy processes, and including means for simply and continuously varying the circuit adjustments so as to enable the circuit to put out an accurately controlled energy level despite wide-range variations in load characteristics; to provide such a circuit in which the circuit adjustment involves simply the adjustment of a variable impedance element, i.e. inductance or capacitance; to provide such a circuit in which the output frequency will remain substantially constant throughout all adjustments; to provide such a circuit that will be simple and economical to make, work and keep up; and that will be readily amenable to automatic control, thereby to permit a more completely automatic operation of the electrical treatments involved than was heretofore believed possible.

I have discovered that an extremely simple and effective means of adjusting the output energy of an oscillator so as to restore the output energy to a desired value whenever a variation occurs in a load impedance, without such adjustment substantially affecting the output frequency, is made possible by coupling the oscillator output with the load through two oscillatory circuits that are coupled with one another beyond their critical coupling factor. Due to the specific properties of such over-coupled circuits, as will be shown in greater detail herein, it becomes possible to adjust the effective power output applied from the oscillator to the load by simple adjustment of ,a variable impedance included in one of the circuits, without substantially affecting the frequency of the output oscillations.

While the type of coupling used between the two circuits may assume any of various formscapable of providing the over-coupled condition specified in accordance with the invention (the term over-coupled being used herein to designate the condition of two circuits so arranged that their mutual coupling factor is beyond its critical value), in a preferred embodiment of the invention there is used a direct form of coupling between the two circuits. Thus the primary circuit may include a capacitance and an inductance connected in series together and both in parallel across the terminals of the secondary circuit, and the latter may include, in addition to the load itself, at least one reactance in parallel with the load and of opposite denomination thereto. The oscillator may then have one output terminal connected to the junction between the capacitance and inductance of the primary circuit, and its other output terminal connected to a point of the secondary circuit, such as a midtap of the reactance in parallel with the load.

Desirably, the requisite adjustment is provided by providing either of the capacitance and inductance elements of the primary circuit adjustable.

Exemplary embodiments of the invention will now be described for illustrative purposes only and with reference to the accompanying drawings, wherein:

FIG. 1 is a simplified electrical diagram of a basic form of the invention in a preferred embodiment thereof;

FIG. 2 shows a family of curves describing the variation of output impedance versus frequency as obtainable with the circuit of FIG. 1;

FIG. 3 is a more detailed diagram of an oscillation generating circuit according to the invention especially applicable to induction heating processes;

FIG. 4 is a graph illustrating various curves describing the variations of certain output characteristics of a circuit such as FIG. 3 as a function of the circuit Q factor;

FIG. 5 is a diagram of another modification of the basic circuit of the invention shown in FIG. 1, especially suitable for dielectric heating processes; and

FIG. '6 shows another form of circuit suitable for induction heating and the like, and provided with selective switching means for changing over between two different output frequencies.

Throughout the circuit diagrams certain circuit elements having comparable functions have been designated with the same reference numbers followed by different letter suffixes.

Referring first to the basic circuit diagram of the preferred form of the invention shown in FIG. 1, a system for generating oscillatory energy according to the invention comprises an electron discharge device shown as a power triode having three electrodes namely a plate 13, a control grid 14 and a cathode 15. It will be understood that means are provided for applying suitable D.-C. voltage'to power the triode, including a plate and a cathode supply. Such means may be entirely conventional and have been omitted from the simplified diagram of FIG. 1. An output circuit is shown as comprising a network of impedance elements including the elements 5, 6 and '7; the'output circuit may be considered as having the pair of input terminals 8 and 9b and the pair of output terminals 3 and 4 connected across the reactive load impedance 1. The load impedance 1 may comprise work, such as a metallic part or a body of material that is to be treated with the oscillatory energy derived from the sysem, and will usually include a resistive component of widely varying value schematically indicated as the series resistance 2. Connected in parallel with the load impedance 1-2 are a first circuit branch comprising an additional reactive impedance 5, and a second circuit branch comprising a pair of further additional reactive impedances 6 and 7 in series. The input terminals of the output circuit means thus provided comprise the point 8 which is the common junction of impedances 6 and 7, and the point 9b shown as a midtap of the impedance 5. Input terminal 8 is connected to the plate 13 of the triode while terminal 9b is connected, as shown through the ground connection 9b-9a, to one terminal of a four-terminal feedback coupling device 10, e.g. an inductive coupling means, having its other input terminal connected to the anode 13 and having its output terminals 11 and 12 connected to the control grid 14 and cathode 15 of the tube respectively.

It will be seen that there has thus been provided a selfsustained oscillation generating circuit in which the output circuit means, i.e. the network connected across the terminals 8-917, actually comprises two separate oscillatory circuits. One is the circuit consisting of the reactive impedances 6 and 7, and may conveniently be termed the primary circuit; and the other, or secondary, oscillatory circuit consists of the load 1-2 with the additional reactance 5 connected across it. With the two oscillatory circuits thus constructed and interconnected at the points 3-4 which are output terminals to the load, both circuits will, in practice, be mutually coupled very tightly and specifically they will be coupled beyond their critical coupling factor; or as this expression is here defined, they are overcoupled, this condition remaining true throughout the whole range of output frequencies. With such an arrangement, assuming the secondary circuit is tuned to a predetermined reference resonant frequency designated F2, it is known that the resonant curve for the primary oscillatory circuit 6-7 will show two separate antiresonance humps, rather than the single peak or hump obtained when the circuits are coupled below or at their critical coupling factor, and that said two humps will be respectively positioned at frequencies below and above the resonant frequency F2 of the secondary circuit.

This condition is illustrated in the graph of FIGURE 2, Where frequencies are plotted in abscissae and output imedances Z are plotted in ordinates. The graph shows a family of three curves which correspond respectively to three different values of the load resistance 2 or circuit Q factor. It will be noted that the frequency scale of the abscissa axis is referred to the frequency F2 of the secondary circuit as the reference or origin frequency. of the three curves shown, curves a, b and 0 correspond respectively to a high, an intermediate and a low value of the load resistance 2. It will be noted that in each of the curves there are two antiresonant humps positioned to opposite sides from the reference frequency F2. The fictive tuned frequency P1 of the primary circuit 6-7 corresponds in each curve with the minimum or valley between the two bumps, and it will be noted that while the three curves a, b and c have been selected for illustrative purposes for conditions such that the said fictive primary tuned frequency F1 is respectively lower than, equal to and higher than the reference frequency F2, still in all three cases there is one antiresonant hump positioned below the reference frequency F2 and the other hump above.

In any one of the curves, the values of the antiresonant frequencies F and F corresponding to the two humps of the curve are determined by the value of load resistance 2 and the relative degree of de-tuning present between the respective tuned frequencies F1 and F2 of the primary circuit 6-7 and secondary circuit 1-5. The value of the load resistance 2 is largely uncontrollable being dependent on the instantaneous characteristics of the work undergoing treatment. However, the tuned frequency F1 of the primary circuit and hence the detuning factor is adjustable through adjustment of any one of the three reactive impedances 5, 6, 7 (conveniently one of impedances 6, 7 provided in the primary circuit), so that the positions of the humps of the curve can be varied. In the operation of the oscillation generator circuit, the frequency produced by the oscillatory element, such as the triode 16, can be made to lock in on either one of the two antiresonant frequencies F, F, say the lower frequency F. Since the ordinate of the antiresonant hump represents the output impedance and hence the output voltage (or energy), it will be clear that through the above mentioned adjustment of the adjustable impedance provided e.g. in the primary circuit, the curve can be deformed so as to bring the particular hump (say hump a) on to the frequency of which the oscillator frequency is locked at the time, to a position tangent to a desired ordinate value representing a prescribed output impedance Z1 and hence a desired amount of output energy. Calculation and experience show that during such adjustment the resulting distortion of the curve occurs in such a manner that the antiresonant frequency F is only very slightly modified; in other words the curve deforms in such a manner that the top of the hump moves up or down in a direction substantially parallel to the ordinate axis with a relatively small amount of sideways shifting.

It therefore becomes evident that with the circuit arrangement shown in FIGURE 1, it is made possible merely through adjustment of a variable capacitor or inductor which may constitute either of the reactive impedances 6, 7 to restore the output impedance and hence the output energy of the system to a prescribed value such as Z1 in FIGURE 2, whenever said output energy has deviated from a prescribed value as upon an uncontrolled variation in the electrical characteristics of the work, and that such compensatory adjustment will only introduce slight variation, in the frequency of the energy applied to the work, so that the desired conditions of treatment are entirely preserved.

As indicated above the adjustment is preferably effected by providing one of the primary reactive impedances 6 and 7 adjustable, and the circuit constants should be so predetermined that adjustment of the adjustable impedance will permit of varying the fictive tuned frequency F1 of the primary circuit over a relatively wide range encompassing the predetermined secondary or reference frequency F2 advantageously over a range extending about from 1/ /2 F2 or 0.7 F2 to /2 F2 or 1.4 F2. In regard to the reactance 5 associated with the load in the secondary circuit, this should be selected so as to impart to the secondary tuned frequency F2 a suitable value as determined by the final output frequency desired, such as F, but such adjustment is not critical.

Usually the reactive load impedance 1 will be such as to present across the output terminals 3 and 4 a reactance in the order of only a few hundredths of the nominal anode load impedance (e.g. Z1) of the oscillator element or tube 16 as required to cause said element 16 to deliver its maximum power rating. Further, with respect to the nominal or maximum anode load impedance Z1, the reactance and capacitance values L and C for the primary circuit impedances 6 and 7 should preferably be so selected that the characteristic primary circuit impedance will only be a relatively small fraction, e.g. no higher than about 8%, of said maximum anode impedance Z1, to ensure suitable stability of the oscillatory frequency. Suitable choice of the various circuit constants in accordance with the preferred teachings or recommendations just set forth lie Well within the capabilities of the average electrical engineer and such choice is not especially critical.

As regards the choice between the two antiresonant frequencies F and F" with either of which the oscillator frequency can be locked, it may be indicated that it will usually be found convenient to select the lower frequency F in cases where the oscillation generator is inductively loaded, and the higher frequency F in case of capacitive loading. It will be understood that the particular frequency F or F" with which the generator frequency will lock is determined by the conditions obtaining at the instant the generator is first started up. That is, the oscillations tend to be set up at the particular one of the frequencies F, F that is situated on the same side of the secondary tuned frequency F2 as is the fictive primary frequency F1. ensure that the system will effectively lock in on the desired one of the frequencies F, F", mean may be provided for only permitting the application of power to the system in case the adjustable reactance 6 or 7 is preset at a value within the range throughout which the just stated condition obtains.

As earlier indicated, the output frequency shift that accompanies a readjustment of the circuit elements 6 or 7 in the operation of the system of the invention, is comparatively minor. Thus, such frequency shift may involve a range of about from 0.8 F2 to 0.9 F2 where the lower antiresonant frequency F is used, or from about 1.1.F2 to 1.25 F2 where the higher frequency F" was selected. This is true even though the fictive primary tuned frequency F1 is variable over the wide range indicated above. Such low shifts in the output frequency of the system during adjustments are usually entirely permissible, especially in intermediate-frequency operation. In cases however where it is desired to ensure an even higher degree of constancy in the output frequency applied to the work, as in R-F operation for eliminating radio interference, this can quite easily be ac- Therefore, in order to complished in the circuit of the invention by providing means for imparting a compensatory shift to the reference or secondary frequency F2 during adjustment of the primary frequency. For this purpose an impedance element of the secondary circuit, such as element 5, may be provided adjustable and servo-means may be provided for automatically adjusting the element in response to departure of the system output frequency from a prescribed value, as later described in greater detail. Such additional adjustment will simply result in introducing a slight and progressive variation in the adjustment range of the primary adjustable element 6 or 7 without any objectionable consequences to system operation. Since the provision of automatic adjusting means of the character referred to is well Within the scope of those skilled in the electronics art, such means are not shown in the drawings.

FIGURE 3 presents a somewhat more detailed diagram of a variation of the preferred system of FIGURE 1, found of especial use in the induction heating of metallic work, e.g. in the production of high-melting alloys, and may comprise a generator of several hundred kilowatt rating designed to operate at audio frequencies. The system includes a power tube 16a having an e.g. watercooled anode 13a grounded at 90. The cathode 15 is heated with current from a high-isolation transformer 17 and is connected through a coupling capacitor 18 to the input terminal 8 of the output circuit means. High gnegative voltage is applied to the cathode by way of choke coil 19 from D.-C. source 20. The control grid 14a of the tube is self-biassed through the parallel resistance-capacitance network 21-22 and is connected to one end of the secondary winding of the feedback coupling transformer 10 the other end of which secondary is connected to the cathode 15a while the primary of the coupling transformed is connected at one end to ground 9b and at its other end to the upper input terminal 8 of the output circuit. Said output circuit includes as the secondary oscillatory circuit therein a fixed capacitor 5a connected across the load inductor 1a so as to .tune the latter to the reference frequencyFZ. The primary circuit portioncomprises a fixed capacitor 6a and an adjustable inductor 7a the inductance of which can be varied over a wide range, e.g. a ratio of one to four, by shifting a magnetic core 24, e.g. of ferrite or the like, relatively to the inductor coil. The load inductor 1a has a midtap connected to ground at 917 to provide the lower input terminal to the output circuit as previously described. Such an arrangement is advantageous in that it halves the effective voltage values requiring to be isolated as between the inductor 1a and the work, here provided in the form of a crucible containing the metal to be melted, as diagrammatically indicated at 24. The system is shown as provided with an alternative load device in the form of a low-impedance inductance coil 25 inductively coupled to coil 1a and having a midtap connected to ground 9b and having its ends 36-37 connected to an external inductor winding 26 adapted to receive further Work therein as shown at 24a, for application of very high-frequency oscillations to the work for induction heating thereof, if required. Capacitor 5a may be replaced or supplemented by a similar capacitor connected across the terminals 36-37.,

In operation, the core or slug 23 is first pushed fully into the inductor coil 7a before applying anode voltage to tube 16a in order to ensure that the output oscillations will lock in on the lower antiresonant frequency F as earlier described. Assuming the work comprising a body of ferromagnetic material in crucible 24 is initially cool, the slug 23 is then pulled out slightly from the coil 7a until the full power in the output oscillations is obtained. The temperature in the metal rises and as the Curie point of the ferromagnetic material is reached the output load of the system tends to drop sharply to a very low value. By gradually pulling out the movable core 23 from inductor 7a the output load can be maintained at its constant maximum value. Similarly any variations in load that may be caused by additional charges of metal introduced into the crucible 24- associated with the alternative inductor device 26 can readily be compensated for by suitably acting on the position of movable core 23. Towards the end of the treatment the core 23 should be gradually pushed back into the inductor 7a to reduce the power output of the generator.

While the above adjustments of the variable inductor element 7a 23 to maintain the desired power output despite variations in output load according to the invention may of course be effected manually, e.g.; in response to temperature readings, it will be understood that they can readily be performed automatically by providing any conventional type of positional servo-system for adjusting core 23 in accordance with controlling factor such as a temperature senser or the like. In this case also suitable automatic adjusting means will be easily provided by those skilled in the art and have not been illustrated.

FIG. 4 illustrates by way of example three characteristic curves describing a typical operation of a generator system according to FIG. ,3. On the abscissae the values of the circuit Q factor of the load inductor la are plotted on a logarithmic scale over a range of values such factor is apt to assume in the practical operation of the system. The V/V-o curve shows the output voltage V required to be produced across output terminals 3 and 4- in order to provide a constant power output equal to the maximum power rating of the system, referred to the voltage V-o across terminals 8&1). Curve L/LIO shows the values of variable inductance Y required for the proper operation of the system at the prescribed constant power rating referred to the value L assumed by the inductance when the primary tuned frequency F1 equals the secondary tuned frequency F2. Curve F lFo refers to the relative variation in true output frequency F over the reference frequency F2. A, B, C, indicate ranges of Q values corresponding to the respective curves at, b, c in FIG. 2.

From an inspection of the curves in FIG. 4 it will be immediately apparent that the system is extremely flexible and capable of maintaining constant output energy conditions over an extremely wide range of conditions. The fact that the L/Lo curve is substantially linear (as referred to the logarithmic abscissa scale) indicates that the adjustment sensitivity of the system remains largely constant throughout its operating range. The low slope of the frequency curve F/Fo throughout said range is also to be noted, indicating the relatively small frequency variations throughout the range adjustment.

FIG. 5 illustrates a further embodiment of the invention especially suitable for the dielectric heating of electrically insulating substances. The load reactance 1b in this case is in the form of a capacitor, means being provided for inserting the material 24 to be treated between the plates of the capacitor. The lower plate 422 is grounded at 90. The additional reactance of the secondary circuit is a fixed inductor 5b connected in parallel with load capacitor 1b. The primary oscillatory circuit here comprises fixed inductor 6b and wide-range adjustable capacitor 71; shown as shunted across the internal capacitance of tube 16!) having a cathode b grounded at 9d. Anode 13b is connected to input terminal 8 of the output circuit by way of a coupling capacitor 18b, and is connected to the positive terminal of high voltage D.-C. source Ztlb by Way of inductor 19b. The control grid 14!) is connected to the cathode by way of selfbiassing R-C network 2Ib-22b in series with a grid inductance 27 which in this modification participates with the internal capacitance of tube 16b to provide the feedback coupling required to provide the sustained oscillations. In the circuit of FIG. 5, the output frequency used is preferably the upper antiresonant frequency heretofore designated F", i.e. the antiresonant frequency above the tuned secondary frequency F2. Accordingly when starting up the system with a cool work charge 24, the adjustable capacitor 712 is preset to a low value and is then increased to build up the output power to its full rated value, and thereafter varied as required, manually or automatically, to maintain said power output despite varying resistance in the material 24 undergoing treatment as earlier explained. For increased output frequency stability, should this be desired, means may be provided for automatically adjusting one of the secondary reactance elements 1b and 5b in response to a frequencymeter reading. For this purpose inductance 5b may be provided with an adjustable magnetic core, and/or as shown, an adjustable capacitor 11) connected in parallel with the load capacitor.

In this regard it is a highly desirable feature of the invention that where there is provided in a system according to the invention a first adjustable reactance such as 712 in the primary circuit and another adjustable reactance such as 28 in the secondary circuit, adjustment of the secondary adjustable reactance 28 results essentially in adjusting the output frequency of the system while adjustment of the primary reactance such as 7b results primarily in adjusting the power output. This feature is advantageous in that it provides a means of adjusting power output and output frequency in a manner substantially independent of each other, and this.

feature is a consequence of the inherent properties of over-coupled circuits used according to the invention, and cannot be achieved with the conventionally used circuits coupled below or at critical coupling.

A third exemplary embodiment of the invention is shown in FIGURE 6, and is suitable for the heat treatment of metal parts especially treatments of the type wherein it is required to preheat the parts to the core prior to surface heating an area that is to be case-hardened. In processes of this kind the preliminary core treatment required the use of relatively low or intermediate frequencies, while the surface heating requires a higher frequency. Heretofore separate apparatus units were required to perform the process, e.g. a motor-driven A.-C. generator for the preliminary low-frequency treatment and an electronic oscillation generator for the high frequency or R-F treatment. While it has been attempted to provide apparatus using common electron power tubes to deliver both frequency ranges involved in such twostep processes, no satisfactory apparatus based on these lines has been developed prior to the invention, to the applicants knowledge, because of the inherent lack of flexibility in the load matching or adjusting means heretofore available. The outstanding flexibility characteristics of the power circuits of the invention have made possible the development of a dual-range power system for two-stage processes of the above mentioned kind, wherein the power output during each stage of the process can be accurately and easily regulated in a corresponding range, and the system can be easily switched over from one operating range to the other; FIGURE 6 shows an example of such a dual-range system according to the invention.

The circuit of FIGURE 6 does not require full description since it is basically equivalent to the circuits precedingly described, its chief distinction lying in the fact that it includes two separate and distinct output circuits each with its primary and secondary oscillatory portions according to the invention, and the two output circuits being selectively and alternatively switchable into the system by the simultaneous actuation of the ganged pairs of switches 3031 and 32-33. The two output circuits are similar and corresponding elements in them are identified by the same numerals followed by sufiix c and d respectively. The output circuit c is switched in for use of the system in delivering radioor high-frequency power, and the output circuit d is used for delivering relatively low (audio or intermediate) frequency power.

The system further includes circuit elements associated respectively with both circuit conditions, but not requiring to be switched in and out owing to the provision of suitable decoupling capacitors. Thus there are provided two different anode choke inductors 19c and 19d and grid inductors 27c and 27d which are, in effect, alternatively in use owing to the provision of the decoupling capacitors 29c29d and 22c-22d respectively. Considering only one of the two circuit conditions of the system, e.g. the one using the lower frequency output circuit d, as obtained with the reverser switches 30-31 and 32-33 in the positions shown, it will be seen that the primary circuit includes the pair of series-connected fixed capacitors 34d-35c respectively of small and large capacitance, and together constituting the capacitance 6d in one series branch and the inductor coil 7d with associated adjustable magnetic core 23d in the other series branch of the primary circuit, with the common junction of said series branches being connected to terminal 8d connected through capacitor 18d and switch arm 30 to the anode 13e of the power tube. The common junctionof the capacitors 34d and 35d is connected through ground to the cathode 15e and through a composite R-C network 21e-22d and grid inductor 27d to the control grid 14e. There is thus provided a Colpitts type oscillator coupling from the output to the power tube. The secondary circuit comprises load reactance 1e formed by the primary winding of a voltage stepdown transformer having the secondary winding 25, the transformer being common to both and d circuit conditions. There is further provided, similar to what is shown in the circuit of FIGURE 3, an alternative load reactance 26 with which an inductive load 24:: is associated. The output circuit made operative with the switches 30-31 and 32-33 displaced to their alternative positions is the same as that described, the elements just mentioned being simply replaced by the elements designated by the same numerals followed by sutfix c instead of d. However, owing to the relatively large ratio between the output frequency values in the respective conditions (a ratio on the order of say 10 to 100), there is shown further associated with one of the two output circuits, specifically the lower frequency or d circuit, a compensating reactance in series with the load, in the form herein of an inductances 38d.

The operation of the system will be evident from the explanations precedingly given. Desirably the power outputs of the respective output circuits are separately preadjusted, and the positional adjustment of the magnetic cores 23c and 23d in each circuit condition, as well as the actuation of switches 30-34 to switch from one to the other condition, may advantageously be controlled automatically in accordance with a preset programme, and/ or in response to means sensing a condition of the load.

Thus it will be apparent that in all of the exemplary systems described, the various objects of the invention are achieved. The systems are usable over a wide range of operating conditions being adjustable throughout said range to maintain a desired power output despite variations in load conditions through the simple adjustment of a variable circuit element and while maintaining a substantially constant, or if desired an accurately constant, output frequency. Their sensitivity ratio remains linear over a wide range.

It is to be distinctly understood that the invention is not limited to the particular circuit configurations shown and described. The oscillator element, shown as a power tube, may be a solid-state element such as a power transistor. While the basic circuit shown in FIGURE 1 and all the remaining circuit embodiments deriving therefrom and shown in FIGURES 3, and 6 utilize a part-icular form of direct coupling between the primary and secondary circuit portions of the output circuit, and said form of coupling is at present preferred since it has given excellent practical performance, other types of coupling,

. 10 including inductive coupling, may be used provided they are capable of ensuring the desired over-coupled condition between the two oscillatory circuit portions of the output circuit, in accordance with the fundamental teaching of the invention.

What is claimed is:

1. An electrical heating system for applying high-frequency electrical energy to work to be heated, which comprises an oscillator having a pair of output terminals and an out-put circuit connected thereto, which output circuit comprises a first resonant circuit portion comprising a first inductive impedance and a first capacitive imped ance in series therewith; a second resonant circuit portion comprising a second inductive impedance and a second capacitive impedance in parallel therewith; one of said second impedances constituting a load impedance arranged in energy-transferring relation with said work;

means coupling said output circuit portions beyond their critical coupling point; a first one of said oscillator output terminals being connected .to a first point of said output circuit between said series-connected first impedances;

the second output terminal being connected to another point of said output circuit; whereby the output energy from said output circuit exhibits a pair of resonance humps for two different frequencies; means providing a positive feedback coupling from said output circuit to said oscillator whereby the natural oscillatory frequency of the oscillator will be determined by the frequency value of one of said humps; and means for adjusting one of said impedances whereby to vary the tuned frequencies of said circuit portions and the peak amplitude of said one hump and hence the amount of energy transferred to the work.

2. The system defined in claim 1, wherein said second output terminal is connected to a point of said second impedances.

3. An electrical heating system for applying high-frequency electrical energy to work to be heated, which comprises an oscillator having a pair of output terminals and an output circuit connected thereto, which output circuit comprises a first resonant circuit portion comprising a first inductive impedance and a first capacitive impedance in series therewith; a second resonant circuit portion comprising a second inductive impedance and a second capacitive impedance in parallel therewith; one of said second impedances constituting a load impedance arranged in ene-rgy tr-ansferring relation with said work; means interconnecting said first impedances with said second impedances so as to couple said resonant circuit portions beyond their critical coupling point; a first one of said oscillator output terminals being connected to a first point of said output circuit between said series-connected first impedances; the second output terminal being connected to another point of said output circuit; whereby the output energy from said output circuit exhibits a pair of resonance humps for two different frequencies; means providing a positive feedback coupling from said output circuit to said oscillator whereby the natural oscillatory frequency of the oscillator will be determined by the frequency value of one of said humps; and means for adjusting one of said impedances whereby to vary the tuned frequencies of said circuit portions and the peak amplitude of said one hump and hence the amount of energy transferred to the work.

4. The system defined in claim 3, wherein the adjustable impedance is one of said first impedances.

5. The system defined in claim 29, wherein the adjustable impedance is one of said first impedances and is adjustable over a range such as to vary the tuned frequency of the first resonant circuit portion over a range extending substantially from about (l/ 2) times to about 2 times the tuned frequency of said second resonant circuit portion.

6. The system defined in claim 3, including means for automatically adjusting said adjustable impedance in response to a departure of an output characteristic of the system from a prescribed value.

7. The system defined in claim 3, including means for preadjusting one of said impedances so as to constrain the natural oscillatory frequency of the oscillator to lock in on the frequency value of a selected one of said humps.

8. The system defined in claim 3, wherein said second inductive impedance constitutes said load impedance, and including an additional inductance inductively coupled with said second inductive impedance and means for supporting said work within said additional inductance to be inductively heated thereby.

9. An electrical heating system for applying high-frequency electrical energy to work to be heated, which comprises an oscillator having a pair of output terminals and an output circuit connected thereto, which output circuit comprises a first resonant circuit portion comprising a first inductive impedance and a first capacitive impedance in series therewith; a second resonant circuit portion comprising a second inductive impedance and a second capacitive impedance in parallel therewith; one of said second impedances constituting a load impedance arranged in energy-transferring relation with said work; means inter-connecting said first impedances with said second impedances so as to couple said resonant circuit portions beyond their critical coupling point; a first one of said oscillator output terminals being connected to a first point of said output circuit between said series-connected first impedances; the second output terminal being connected to another point of said output circuit; where by the output energy from said output circuit exhibits a pair of resonance humps for two different frequencies; means providing a positive feedback coupling from said output circuit to said oscillator whereby the natural oscillatory frequency of the oscillator will be determined by the frequency value of one of said humps; and means for adjusting one of said first impedances whereby to vary the tuned frequencies of said circuit portions and the peak amplitude of said one hump and hence the amount of energy transferred to the work, and means for adjusting one of said second impedances whereby to vary the output frequency of the system.

10. The system defined in claim 9, including first means for automatically adjusting said adjustable first impedance in response to a departure of the energy output of said system from a prescribed value, and second means for automatically adjusting said adjustable second impedance in response to a departure of the output frequency of the system from a prescribed frequency value.

References Cited by the Examiner UNITED STATES PATENTS 2,470,443 5/49 Mittelmann 219-10.77 2,684,433 7/54 Wilson 21910.77 2,765,387 10/56 Wilson 21910.77 X

FOREIGN PATENTS 573,496 4/59 Canada. 946,841 6/49 France.

OTHER REFERENCES Radio Engineering, by Frederick Emrnons Terman, Third Edition, McGraw-Hill Book Company, 1947.

RICHARD M. WOOD, Primary Examiner. 

1. AN ELECTRICAL HEATING SYSTEM FOR APPLYING HIGH-FREQUENCY ELECTRICAL ENERGY TO WORK TO BE HEATED, WHICH COMPRISES AN OSCILLATOR HAVING A PAIR OF OUTPUT TERMINALS AND AN OUTPUT CIRCUIT CONNECTED THERETO, WHICH OUTPUT CIRCUIT COMPRISES A FIRST RESONANT CIRCUIT PORTION COMPRISING A FIRST INDUCTIVE IMPEDANCE AND A FIRST CAPACITIVE IMPEDANCE IN SERIES THEREWITH; A SECOND RESONANT CIRCUIT PORTION COMPRISING A SECOND INDUCTIVE IMPEDANCE AND A SECOND CAPACTIVE IMPEDANCE IN PARALLEL THEREWITH; ONE OF SAID SECOND IMPEDANCES CONSTITUTING A LOAD IMPEDANCE ARRANGED IN ENERGY-TRANSFERRING RELATION WITH SAID WORK; MEANS COUPLING SAID OUTPUT CIRCUIT PORTIONS BEYOND THEIR CRITICAL COUPLING POINT; A FIRST ONE OF SAID OSCILLATOR OUTPUT TERMINALS BEING CONNECTED TO A FIRST POINT OF SAID OUTPUT CIRCUIT BETWEEN SAID SERIES-CONNECTED FIRST IMPEDANCES; THE SECOND OUTPUT CIRCUIT; WHEREBY THE OUTPUT ENERGY POINT OF SAID OUTPUT CIRCUIT; WHEREBY THE OUTPUT ENERGY FROM SAID OUTPUT CIRCUIT EXHIBITS A PAIR OF RESONANCE HUMPS FOR TWO DIFFERENT FREQUENCIES; MEANS PROVIDING A POSITIVE FEEDBACK COUPLING FROM SAID OUTPUT CIRCUIT TO SAID OSCILLATOR WHEREBY THE NATURAL OSCILLATORY FREQUENCY OF THE OSCILLATOR WILL BE DETERMINED BY THE FREQUENCY VALVE OF ONE OF SAID HUMPS; AND MEANS FOR ADJUSTING ONE OF SAID IMPEDANCES WHEREBY TO VARY THE TUNNED FREQUENCIES OF SAID CIRCUIT PORTIONS AND THE PEAK AMPLITUDE OF SAID ONE HUMP AND HENCE THE AMOUNT OF ENERGY TRANSFERRED TO THE WORK. 